1. Field of the Invention
This invention relates to a numerical control (NC) apparatus including a plurality of driving shafts for moving a table supporting a workpiece member in three dimensional directions, and a spindle for supporting a tool. More particularly, this invention relates to a NC control apparatus wherein profile machining is made by utilizing arc interpolation technique, the position error of the spindle axis can be optimumly corrected at a high accuracy when the directions of movement of respective driving shafts are changed.
2. Description of the Related Art
In a numerically controlled machine tool for performing three dimensional cutting or the like, there are provided driving shafts movable in X, Y and Z directions of rectangular coordinates, driving shaft control apparatus for respective driving shafts and a spindle for mounting a tool. These driving shaft control apparatus are controlled simultaneously for moving a table on which a workpiece is mounted for machining the workpiece three dimensionally. For effecting such multi-shaft simultaneous control a programmed NC control apparatus is generally used. Since the numerically controlled machine tool described above is well known in the art, for the sake of brevity it is not shown in the drawing.
In the driving shaft control apparatus described above, each apparatus is constituted by a servomotor for moving a movable element for carrying a workpiece, servo control means for driving the servomotor in response to a position instruction, and a position detector for detecting the position of the servomotor for feeding back the detected position to the servo control means. These mechanisms are provided for all driving shafts.
In such NC control apparatus, there occurs various position errors due to the presence of mechanical parts, among which backlash error is a typical position error. Various methods have been proposed for compensating for the position error in the directions of movement of respective drive shafts, that is the position error occurring at the time of feeding the movable elements in the same direction. Such prior art methods are disclosed in the Japanese Laid Open patent specification Nos. 3086/1988 and 2074/1977.
In the prior art apparatus, however, a small position error of the spindle occurs when the direction of movement of each drive shaft is reversed.
For example, where the directions of movement of respective drive shafts in the three dimensional directions are reversed while feeding the table of a horizontal type machining center, a small error in parallel with the axis of the spindle occurs, but in the prior art control apparatus such position error has not been corrected.
FIG. 3 is a diagram showing the position error of a horizontal type machining center wherein the axis of the spindle is parallel to the Y axis of table driving axis, the position error being measured by double ball bar test method (DBB), this method being described in detail in a paper entitled "Study on the Motion Accuracy of NC Machine Tools (part 1), described by Yoshiaki Kakino and presented to the spring meeting of the Institute of Precision Machining, held on Sep. 2, 1985 and Method of Evaluation of the Degree of Precision of NC Machine Tool, published by Murata, Sep. 20, 1989, page 20, Chapter 3 "Measuring Apparatus".
FIG. 3 is a diagram showing the result of measurement obtained by measuring the position error of a horizontal type machining center of the table driving shaft wherein the spindle axis is parallel to the Y axis of the table drive shaft by using the DBB method. In FIG. 3, X and Y represent the X and Y coordinate axes of the direction of movement of the table carrying the workpiece, in which the axis of the spindle is parallel with the Y axis.
In a case involving a curved workpiece, for example, a cylinder, when the table is moved from a position 1 in the Y axis direction to position 3 in the Y axis direction through a position 2 in the X axis direction by using arc interpolation method, the direction of movement of the table in the direction of Y axis is caused to reverse from position 3. At this time, due to a mechanical error, the axis of the spindle is caused to vary in a X axis direction 2. As a consequence, the axis of the spindle would move to the starting point 1 through a deviated position 4 in the X axis direction.
Thereafter when the table is started to rotate in the counterclockwise direction from point 2 in the X axis direction, since the direction of movement of the table in the Y axis direction is reversed, the position of the axis of the spindle would become erroneous in the direction 2 along the X axis. Thus, the table does not pass through point A in the case of clockwise rotation, but instead the table would begin to start from position B to travel from position 1 on the Y axis, and is moved toward position B. Thus, the table would be moved from position 1 in the Y axis to position 4 in the X axis. Under these conditions, the movement of the table in the Y axis direction is reversed thereby causing a position error of the spindle for the same reason as above described.
Although this error reaches a maximum value when the direction of movement of the drive shaft parallel to the spindle axis is reversed. In the other drive shafts too, similar errors occur when the directions of movements of these drive shafts are reversed.
Since these phenomena are repeated, due to the reversal of the direction of movement of the drive shaft when it is moved in the clockwise direction as well as in the counterclockwise direction, a position error of the spindle axis occurs with the result that the locus of the movement of the spindle will become a double circle as shown by symbols CW and CCW in FIG. 3, thus decreasing the accuracy of the movement of a NC controlled mechanism, and the accuracy of positioning thereby degrading the machining accuracy.
The defects of the prior art NC control apparatus described above will be summarized as follows with reference to FIG. 5. A position instruction Xc is supplied to a servomotor 103 via a first adder 101 and a servo control apparatus 102. The number of rotations or an angular position of the rotor of the servomotor 103 is sensed by an encoder 104 and the output thereof is supplied to one input terminal of a second adder 105. In response to the position instruction Xc, a reversal detector 106 issues an output signal in response to the reversal of the position instruction Xc, and the output signal of the reversal detector 106 is applied to one stationary contact of a transfer switch 107, the stationary contact being normally applied with a zero backlash signal 0. The other stationary contact of the transfer switch 107 is applied with a compensation data Ax and the output of the transfer switch 107 is applied to the second adder 105. The sum or difference obtained by the second adder 105 is supplied to the other input terminal of the first adder 101 which functions to minimize the position error of the movable member.